This invention relates to a recording/reproducing apparatus and a recording/reproducing method for optical recording media, particularly to a recording/reproducing apparatus and a recording/reproducing method for recording and/or reproducing information on/from each of said optical recording media via two types of lenses (two-group objective lenses).
In order to increase the recording density of an optical disk, it is necessary to narrow the pitch between marks for recording information and reduce the size of the optical spot (hereafter, to be referred to as the spot simply). The optical spot size can be represented by .lambda./(2NA) if the object light beam wave length is defined as .lambda. and the objective lens numerical aperture as NA. If the light beam wave length .lambda. is fixed, therefore, the larger the numerical aperture NA becomes, the smaller the spot size becomes.
The optical disk apparatus that is put to practical use at present uses a single objective lens and its numerical aperture (NA) is limited to 0.6 for the manufacturing reason of the aspheric surface lens used as the objective lens.
To cope with this problem, a method was proposed, for example, by S. M. Mansfield, W. R. Studenmund, and K. Osato in "High-Numerical-Aperture Lens System for Optical Storage", Opt, Left. 18, 305-307 (1193)" for forming two-group objective lenses using two lenses so that the numeric aperture NA can exceed 0.8. Another method for using a solid immersion lens was also disclosed in U.S. Pat. No. 5,125,750.
If such two-group lenses are used, however, the air gap between one of the lenses, which is closer to an optical disk, and the optical disk itself is changed, the change causes spherical aberration to occur and accordingly deteriorate regenerated signals. In the worst case, it becomes impossible to record/reproduce information on/from the object optical disk.
When recording or reproducing information on/from another optical disk having a different thickness of light transmission substrate after locations of the two-group lenses are optimized for an optical disk having a specified substrate thickness, spherical aberration occurs due to the difference of the disk substrate and it becomes impossible recording or reproducing the information correctly.
Furthermore, if, even when recording or reproducing information on/from an optical disk having two or more recording layers that are laminated, locations of the two-group objective lenses are optimized for a specific recording layer, then the information cannot be recorded or reproduced on/from another recording surface correctly due to spherical aberration caused by a layer that separates each recording layer (ex., UV range layer).
Under such circumstances, it is an object of this invention to improve the recording density of optical disks and suppress occurrence of spherical aberration caused by using another optical disk having a different substrate thickness or by using a different recording layer of the optical disk.